1. Field of the Invention
The invention relates to the field of the method of extruding thermoplastic resin pipes which are utilized in gas supply or other general purpose areas.
2. Description of the Related Art
In the conventional method wherein thermoplastic pipes are extruded, the deformation is carried out mainly in the axial direction. The material is elongated to the axial direction, which causes the molecular chain of the resin to be oriented in that direction. As the result of the extrusion, the property of the pipe is not uniform wherein the strength of the pipe in the direction of the axis is high and the strength thereof in the direction of circumference is low.
Japanese Patent laid open 36628/1982 discloses a method of preventing the deterioration of the property of the resin pipes. FIG. 4 is an exemplary view of the apparatus in a conventional method. As shown in FIG. 4, the tubular body, extruded by the die 42 installed at the outlet of the extruder 41, is transferred to the blow drawing device 45, which continuously draws the tubular body, and which applies the force of expansion on the tubular body by sucking the air surrounding the outer surface of the body. The step is called "blow drawing". The drawn body is transferred by the taking up machine 46.
FIG. 2 is a sectional side view of the material flow of the resin in the conventional method. In FIG. 2, 1 denotes the die; 2, the outer diameter of the die lip; 3, the thickness of the die lp; 4, the sizing device; 5, the inner diameter of the sizing device; 6, the outer portion of the die; 7, the inner portion of the die. The die lip encompasses the body being extruded with the die at the outlet of the die. In the conventional method the outer diameter of the die lip 2 is the same as or larger than the inner diameter 5 of the sizing device 4. The shape of the die lip is determined by a clearance formed between the outer portion of die 6 and the inner portion of die 7.
The percentage of the difference between the outer diameter of the die lip and the inner diameter of the sizing device, divided by the outer diameter of the die lip is called the reduction ratio.
In the conventional method, the higher the speed of the extrusion the worse the property of the resin pipe. Moreover the greater the reduction ratio the worse the property of the resin pipe. FIG. 3 graphically shows the relationship between the take up rate and fracture elongation for pipes extruded at varying reduction ratios. The abscissa denotes the take up rate or the drawing speed, and the ordinate denotes fracture elongation. The percentage referring to each curve denote the reduction ratio. Generally speaking a value of fracture elongation of 125% is the lowest limit of the guaranteed quality. As shown in FIG. 3, the higher the drawing speed and the higher the reduction ratio, the worse the rupture elongation.
Japanese Patent laid open 36628/1982 reports that the inherent elongation property of the resin pipe is lowered since the pipe possesses a residual stress due to the deformation in the axial and circumferential directions in the production process. When the pipe is under internal pressure, a slitting rupture or tearing off may occur. When the pipe is utilized for a long duration of time, or warm water flows in the pipe, the dimensional change due to the shrinkage of the pipe or a crack rupture due to the stress concentration at the fixed part may occur. When the pipe is bent with a small curvature, a bending stress is generated which leads to a crack rupture or a buckling.